This paper examines the physiological mechanisms responsible for differences in the amplitude of force fluctuations between young and old adults. Because muscle force is a consequence of motor unit activity, the potential mechanisms include both motor unit properties and the behavior of motor unit populations. The force fluctuations, however, depend not only on the age of the individual but also on the muscle group performing the task, the type and intensity of the muscle contraction, and the physical activity status of the individual. Computer simulations and experimental findings performed on tasks that involved single agonist and antagonist muscles suggest that differences in force fluctuations are not attributable to motor unit twitch force, motor unit number, or nonuniform activation of the agonist muscle, but that they are influenced by the variability and common modulation of motor unit discharge in both the agonist and antagonist muscles. Because the amplitude of the force fluctuations does not vary linearly with muscle activation, these results suggest that multiple mechanisms contribute to the differences in force fluctuations between young and old adults, although the boundary conditions for each mechanism remain to be determined.

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